Method to limit a creep of bolts and gaskets of bolted flanged connections

ABSTRACT

A method to limit or exclude operational creep of the bolts and gaskets of Bolted Flanged Connections (BFCs) is disclosed. The method consists in the use of bolts and gaskets manufactured from the same Shape Memory Alloys (SMAs) having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs. These bolts and gaskets of the BFCs are shape-memorized to the compression (bolts) and “swelling” (gaskets) during (1) the formation of stress-induced martensite under temperature of martensite state of the SMAs while bolt preload application, or (2) during the formation of stress-induced martensite while loading-unloading previously stretched bolts and previously compressed gaskets under temperature of martensite state resulting in residual bolt elongations and residual gasket contraction followed by bolt preload application. Constrained shape recovery of initial length of the bolts and initial thickness of the gaskets under operational temperatures of the BFCs is accompanying by appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of the bolts and gaskets that limits or excludes their operational creep.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Patent Application is continuation of earlier provisional patentapplication Ser. No. 60/925,840 filed Apr. 24, 2007, and patentapplication Ser. No. 10/834,955 filed Apr. 30, 2004 now abandoned.

FEDERALLY SPONSORED RESEARCH

None

SEQUENCE LISTING OR PROGRAM

None

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to Bolted Flanged Connections (BFCs) with gasketsas sealing elements that must provide a leak-tight and durable jointbetween component parts of the pressure vessels, piping systems, andother engineering structures that operate under conditions of internalpressures and a variety of operational temperatures.

2. Background of the Invention

One of the most typical means to obtain a leak-tight and durable jointbetween component parts of boilers, reactors, steam generators, pipingsystems, and other engineering structures is an assemblage of theirpieces with the BFCs. These types of connections have a wideapplicability in petrochemicals, petroleum refining, aerospace,submarine shipbuilding, fossil fuel and nuclear power generation, andother critical industries.

There are millions of bolts and gaskets used in critical engineeringstructures, and the problem of the BFCs' structural integrity andplant/piping operational leakage reduction is very complex and involvesmany areas of applied mechanics and technological findings. Safe designof the BFCs from structural integrity point of view has beensatisfactorily solved and standardized, but the leakage events remain anunresolved problem and a principal cause of bolt and/or gasket damagesand failure that are attributed to the high level of corrosion combinedwith high level of stresses and strains due to cyclic internalpressures, external loadings, elevated operational temperatures,flow-induced vibrations, integral flow of neutrons, and other criticalfactors.

Statistic data show that, for example, piping system leakages aloneconservatively cost each process industry hundreds millions of dollarsannually in lost profits as a result of plant shutdowns, productionpenalties, maintenance rework activities, equipment repair orreplacement, and so on. It is necessary to add the liquidation ofconsequences of possible fires, explosions, environmental pollution, andother disasters. Hence, the leak tightness of the BFCs used in criticalplant/piping systems has a greater influence on their safe and extendedservice life, and highest priority in plant/piping reliability programsis to limit or exclude the operational leakages, thus protectingcritical engineering structures from untimely degradation and failure.

One of the main causes of plant/piping leakages is a “passive” behaviorof bolts and gaskets under critical operational conditions whennonlinear gasket response to the loading-unloading processes combinedwith creep of bolt and gasket materials lead to clamping force decreaseand unavoidable joint opening followed by leakages. Generally, creep isaccompanying by stress relaxation, and elongation of the bolts alongwith contraction of the gasket due to creep-relaxation is a very seriousproblem because it lead to bolt load and gasket stress losses that, inturn, increase the leakage rate.

It is known that creep-relaxation of the bolts and gaskets increaseswith elevated temperatures and load-induced stresses although roomtemperature creep-relaxation can be also significant even at relativelylight loads. A plant maintenance practice includes periodicalretightening or replacement of the bolts and gaskets subjected tocreep-relaxation to prevent leakages, and, having in mind a greatquantity of bolts and gaskets used in critical industries, the procedureinvolves an expensive time-consuming process, but it provides onlytemporary effect because the creep-relaxation increases rapidly aftereach additional retightening and the risk of leakage event relativelyincreases. Nevertheless, during the development of most designprocedures, a little consideration has been given to operationalcreep-relaxation of the bolts and gaskets, and similar situation isobserved with patent documents.

U.S. Pat. No. 6,199,453 by Steinbock entitled “High temperature boltingsystem” offers a sophisticated apparatus for maintaining a clamping boltforce between component parts of a steam turbine while operating attemperatures from 800° F. to 1200° F. However, the disclosed elongatedstepped fastener shank manufactured from superalloy Inconel 718 having athermal expansion coefficient similar to flange material cannot stop acreep-relaxation process and protect proposed bolting system fromcreep-relaxation that is an increase of bolt elongations and decrease ofbolt stresses with time. Moreover, the combination of high level ofstresses and temperatures will induce the high level of creep-relaxationof the bolts that defines their routine “passive” behavior undercritical operational conditions.

EP Pat. No. 352608 discloses a method of fabrication of reinforcedpolytetrafluoro-ethylene (PTFE) gasketing materials “characterized byhigh strength, excellent recovery and superior creep-relaxationresistance”. However, these super characteristics did not exclude thecreep-relaxation from 20% to 30% in dependence on thickness of proposedtested materials. Moreover, the tests were carried out under standardprocedure during only 22 hours at only 212° F. Thus, the proposedgasketing materials demonstrate “superior” physical and functionalproperties when compared with previous PTFE gasketing materialsdescribed by cited prior art. The most important failing however is thefact that proposed gasketing materials copy a typically used approach tothe fabrication of any known sealing materials based on theirtraditional “passive” behavior under critical operational conditions.

The attempts to use the bolts and gaskets manufactured from advancedShape Memory Alloys (SMAs) were made in Japanese patent publicationsNos. 59026668, 62188764, 63172064, 1255782, 4073469, 6101762, 2005249123as well as in GB2352768, and U.S. Pat. Nos. 5,226,683 and 6,435,519.

JP 59026668 describes a gasket obtained by bending a plate material ofSMA in an annular body having U-shaped cross sectional form. The gasketis so set that its width in the axial direction becomes larger than thedepth of a fitting groove in a temperature range higher than the shapememory temperature, and becomes smaller than the depth of the groove ina temperature range lower than the shape memory temperature providing inthe first case a sealing action between the members of the assembly.This invention relates to the use of two-way shape memory effect thatmay be completed under condition of preliminary continuous training ofthe gasket at two temperature ranges.

JP 62188764 discloses a method to manufacture a bolt of NiTi SMA thatmay be easily fastened and detached. This bolt is previously subjectedto axial compression and aging treatment under specific high temperaturewhile holding it under compressive strain. Thus-obtained bolt repeatsreversibly the elongation in a length direction at a temperature ofmartensite transformation and the contraction at a temperature ofinverse transformation. Owing to these characteristics, the length ofthe bolt is arbitrary changed, so that bolt may be firmly fastened oreasily detached. Obviously, this invention relates to the assemblageprocedure of the BFC based on two-way shape memory effect undertemperatures corresponding to martensitic phase transformation of NiTiSMA. Additionally, the bolt fabrication procedure is based oncompression of the bolt under high temperature of austenite state thatincreases a risk of the bolt's buckling.

JP 63172064 describes a gasket of SMA that remains in nearly flat shapeat low temperature and forms a bead on the peripheral edge part ofopening to be sealed when its temperature becomes above a defined value.This invention is close to the present invention, but the problem ofcreep-relaxation of the gasket is completely failed.

JP 1255782 relates to a gasket of SMA that is extended or contracted byheating or cooling under preset temperatures. The gasket body is heatedby a heater, and it is expanded to form a fully tight structure. Theremoval of the gasket is performed by cooling that contracts it. Thisinvention relates to the well known assemblage procedure of the BFCdescribed by some mentioned above patent publications.

JP 4073469 describes a gasket of SMA inserted between the sides of theflanges where the knife edges are formed. The gasket is heated withheaters inserted into the bodies of the flanges and restores theoriginal shape ensuring higher tightness. When the gasket is releasedfrom connection and heated with another heating device, a recess due tothe knife edge can be eliminated and the original shape of the gasketcan be restored. Consequently, the gasket can be again used.

JP 6101762 proposes a spiral-shaped gasket of SMA that is set betweenthe flanges, and sealing effect is performed even if the tighteningforce of the flanges and bolts lowers because of thermal expansion. Thisinvention is very close to the one described above by JP 58180892.

JP 2005249123 describes the bolts from CuAlNi and FeNiCoTi SMAs. Theexpanded shape of the bolts is stored at normal temperature, andcontracted shape is stored at high temperature. The fastening andunfastening work can be easily performed under normal temperature, andstrong fastening force is exercised under high temperature. Thisinvention relates to well known assemblage procedure of the BFC.

GB 2352768 proposes the bolts of NiTiCuFe SMA that connect the membersof cryogenic satellite tank when the bolts are in their austenite state,but connection between the members is loosened when the bolts expand intheir martensite state following cooling in space. This inventionrelates to assemblage-disassemblage procedure of the BFC.

U.S. Pat. No. 5,226,683 discloses a method to use a gasket of NiTi SMAunder martensite state in order to fill the microspaces between the hardflange faces having microscopic surface irregularities that can preventthe fluid leakage between the faces and will allow further to reuse thegasket. The NiTi SMA of which the gasket is made “remembers” itsoriginal shape when it was formed in its austenite state. When thisgasket is deformed under temperature of martensite state it fills theirregularities of flange faces under pressure exerted by hard clampingflanges. The shape memory effect is used when gasket resumes itsoriginal shape being released and heated to austenite state during therestoration step before reuse.

U.S. Pat. No. 6,435,519 presents a next attempt to use the NiTi SMA as agasketing material in order to provide a seal between component parts ofan imaginary generalized assembly. Unfortunately, this invention claimsa well-known procedure to clamp the gasket between adjacent flangefaces. As for application of the SMA, this invention claims a springforces generated by bending of the gasket when it is in super-elasticstate. It is easily to observe that SMA in its super-elastic statedisplays all mechanical properties of typical elastic material includingthe property of creep-relaxation while subjecting to elevatedtemperature and internal pressure.

Clearly, none of above-mentioned prior patent documents touch theproblem of leakage elimination by means of creep limitation of the boltsand gaskets of the BFCs. Accordingly, it is a principal object of thepresent invention to form a method of creep limitation of the bolts andgaskets that provides an “active” behavior of the bolt-gasket systemunder critical operational conditions including internal pressures and avariety of operational temperatures. This invention is the first oneintroducing a new sealing philosophy based on active intervention of thebolts and gaskets into sealing process under critical operationalconditions. This active intervention is called “negative creep” effectto satisfy the conditions as employed in the present invention.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to introduce anew sealing technology based on active intervention of bolts and gasketsof the BFCs into technological processes of engineering structures thatoperate under critical conditions due to variety of operationaltemperatures and internal pressures.

It is next object of the present invention to provide activeintervention of bolts and gaskets of the BFCs into criticaltechnological processes by means of significant limitation or completeexclusion of their operational creep.

It is another object of the present invention to provide a method ofcreep limitation of the bolts and gaskets by means of specific types ofbolts and gaskets that are manufactured from SMAs and display “negativecreep” effect under critical operational conditions. “Negative creep”effect of the bolts and gaskets results from reactive shape-recoveringstresses that appear during constrained shape recovery ofshape-memorized deformations of the bolts and gaskets under operationaltemperatures. Reactive shape-recovering stresses have direction inverseto the direction of operational creep of the bolts and gaskets thatlimits or excludes the bolt elongations and gasket contraction due tocreep.

Method of creep limitation consists in application of bolts and gasketsmanufactured from the same SMAs having temperature intervals of reversemartensitic phase transformation close to operational temperatures ofthe BFCs. The bolts and gaskets of SMAs are shape-memorized to thecompression (bolts) and “swelling” (gaskets) by different ways includingbolt preload force that produces elongation of the bolts and contractionof the gasket. These deformations are shape-memorized ones because theychange the initial length of the bolts and initial thickness of thegasket. The bolts and gasket that are shape-memorized by bolt preloadforce will try to recover their initial length and initial thicknessunder operational temperature of the BFC, but this shape recovery willbe blocked by rigid flanges with appearance of reactive shape-recoveringstresses having direction inverse to the direction of operational creepof the bolts and gasket. Reactive shape-recovering stresses will blockthe bolt elongations and gasket contraction due to creep thatcorresponds to “negative creep”

The shape-memorized deformations of the bolts and gasket due to boltpreload force can be combined with their residual shape-memorizeddeformations obtained in advance during the formation of stress-inducedmartensite. Stress-induced martensite is formed while loading-unloadinga work piece of SMA under temperature below than temperature ofmartensite state of the SMA, and this procedure results in appearance ofresidual shape-memorized deformation of the work piece that can be thebolt or gasket.

The recovery of initial length of the bolts and initial thickness of thegasket occurs after standard tightening of the bolts and clamping of thegasket followed by application of internal pressures and a variety ofoperational temperatures that are close to the temperatures of reversemartensitic phase transformation of the SMAs from which the bolts andgaskets are manufactured.

The SMAs on a basis of Cu, Fe, Al, Ma, Co, Ga, In, Ni, Ti, Zr, Pd, Pt,Hf, and other elements have large temperature intervals of reversemartensitic phase transformations that correspond to the variety oftemperatures which provide the recovery of shape-memorized deformations.Hence, the operational temperature of the BFC must be close totemperature interval of reverse martensitic phase transformation of theSMA from which the bolts and gasket are manufactured. For example, theoperational temperatures of technological equipment such as rectors,steam generators, heat exchangers, piping systems, and others used inFossil Fuel and Nuclear Power Plants vary from dozens to hundredsdegrees, and usually they remain stable for given type of equipment, sothat temperature of reverse martensitic phase transformation of suitableSMA must be in the interval of operational temperatures of the assembly.This condition ensures the recovery of initial length of the bolts andthickness of the gasket, but this shape recovery will be blocked byrigid flanges of the BFC with appearance of reactive shape-recoveringstresses having direction inverse to the direction of operational creepof the bolts and gasket that corresponds to “negative creep” effect.

Reactive shape-recovering stresses may be considerable depending onquantity of shape-memorized deformations and rigidity of opposed flangesof the BFC that provides significant limitation or inhibition ofoperational creep of the bolts and gasket. For example, reactiveshape-recovering stresses of the SMAs on a basis of Ni—Ti compositionsmay be close to 800 MPa, and for some compositions of Ni—Ti—Hf they canreach 1300 MPa.

A principal advantage of the present invention is a new approach to thesealing technology based on “active” intervention of bolts and gasketsof the BFCs into technological processes by means of bolts and gasketsmanufactured from SMAs having temperature intervals of reversemartensitic phase transformation close to operational temperatures ofthe BFCs.

Another advantage of the present invention consists in the use ofreactive shape-recovering stresses generated by constrained recovery ofshape-memorized deformations of the bolts and gaskets manufactured fromSMAs and shape-memorized to the compression (bolts) and “swelling”(gaskets). The reactive shape-recovering stresses appear underoperational temperatures of the BFCs providing the “negative creep”effect that limits or excludes operational creep of the bolts andgaskets.

It is a next advantage of the present invention to provide a continuousautomatic tight contact between flange and gasket surfaces undercritical operational conditions including high internal pressures and avariety of operational temperatures. This contact is provided byconstrained shape recovery of the gasket thickness of SMA that isshape-memorized to the “swelling”. Reactive shape-recovering stressesdue to constrained recovery of initial thickness of the gasket ensurethis contact that significantly increases a leak-tightness of the BFCand maximizes efficiency of critical technological equipment.

Further brief description of applied drawings followed by detaileddescription of the invention is intended to provide a necessary basisfor understanding the nature and character of the present invention, andintroduce the method to limit a creep of bolts and gaskets of the BFCs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a part of the BFC with the bolts andgasket of the same SMA having temperature interval of reversemartensitic phase transformation from A_(s) (start temperature) to A_(f)(final temperature).

FIG. 2 represents schematically constrained shape recovery of the boltof SMA that is shape-memorized to the compression by bolt preload forceproviding contact shape-memorized deformation ε_(c) under temperaturebelow than temperature M_(f) of martensite state of the SMA. Constrainedshape recovery of the bolt is accompanying by appearance of reactiveshape-recovering stress σ_(sr) at temperature interval A_(s)≦T≦A_(f) ofreverse martensitic phase transformation of the SMA.

FIG. 3 represents schematically constrained shape recovery of the boltof SMA that is shape-memorized in advance to the compression during theformation of stress-induced martensite while loading-unloading the boltunder temperature below M_(f) followed by application of bolt preloadforce that provides contact shape-memorized deformation ε_(c) due tostress σ_(c). The constrained shape recovery is accompanying byappearance of reactive shape-recovering stress σ_(sr) at temperatureinterval A_(s)≦T≦A_(f) of reverse martensitic phase transformation ofthe SMA.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of a part of BFC having the flanges 1and 2 of conventional structural steel, and bolts 3 and gasket 4manufactured from the same SMA having temperature interval A_(s)≦T≦A_(f)of reverse martensitic phase transformation that is close to operationaltemperatures of the BFC. Bolts and gasket are shape-memorized to thecompression (bolts) and “swelling” (gasket). The constrained shaperecovery of stretched bolts and compressed gasket at temperatureinterval A_(s)≦T≦A_(f) produces reactive shape-recovering stressesσ_(sr) having direction inverse to the direction of operational creep ofthe bolts and gasket. Hence, FIG. 1 illustrates a main principle of“negative creep” effect that limits or excludes elongation of the boltsand contraction of the gasket due to their operational creep undertemperature interval A_(s)≦T≦A_(f) of the BFC.

FIG. 2 is a schematic representation of constrained shape recovery ofthe bolt of SMA that is stretched by bolt preload force from point O topoint K under temperature below than temperature M_(f) of directmartensitic phase transformation of the SMA producing contactshape-memorized deformation ε_(c) due to stress σ_(c). This procedure isshown at the rear graph of “stress σ—strain ε—temperature T” diagram.The recovery of initial length of the bolt constrained by rigid flangesoccurs under temperature interval A_(s)≦T≦A_(f) that produces thereactive shape-recovering stress σ_(sr) while heating the BFC from pointK to point H shown at the front graph in FIG. 2. Similar procedure maybe applied to the gasket compressed by bolt preload force under the sametemperature conditions. In this case it is sufficient to changedirections of the axes σ and ε. Reactive shape-recovering stress σ_(sr)will block the bolt elongations and gasket contraction due to theiroperational creep that corresponds to “negative creep” effect.

FIG. 3 is a schematic representation of constrained shape recovery ofthe bolt of SMA that is shape-memorized in advance to the compressionduring formation of stress-induced martensite while loading from point Oto point K followed by unloading from point K to point L undertemperature below M_(f). After this procedure bolt receives somequantity of residual elongation defined by location of the point L.Application of bolt preload force leads to additional bolt elongationfrom point L to point G producing final contact shape-memorizeddeformation ε_(c) due to stress σ_(c). This procedure is shown at therear graph in FIG. 3. The recovery of initial length of the boltconstrained by rigid flanges occurs under temperature intervalA_(s)≦T≦A_(f) that produces the reactive shape-recovering stress σ_(sr)while heating the BFC from point G to point H shown on the front graphin FIG. 3. Similar procedure may be applied to compressed gasket thatreceives final contact shape-memorized deformation ε_(c).

The shape-memorized deformation of the bolts and gaskets obtained withdescribed procedures are a basis to produce “negative creep” effectunder operational temperatures of the BFCs resulting in appearance ofreactive shape-recovering stresses having direction inverse to thedirection of operational creep of the bolts and gaskets that limits orexcludes their operational creep. This forms a basic idea of presentedmethod.

CONCLUSION

Bolts and gaskets, manufactured from SMAs having temperature intervalsof reverse martensitic phase transformation close to the operationaltemperatures of the BFCs and shape-memorized to the compression (bolts)and “swelling” (gaskets), display “negative creep” effect resulting inappearance of reactive shape-recovering stresses due to constrainedrecovery of initial length of the bolt and initial thickness of thegaskets under operational temperatures of the BFCs. The reactiveshape-recovering stresses having direction inverse to the direction ofoperational creep of the bolts and gaskets limit significantly orexclude completely bolt elongations and gasket contraction due to theiroperational creep.

Described procedures of “negative creep” formation are the basis of newsealing technology and method to limit operational creep of bolts andgaskets of the BFCs that is a principal object of the present invention.This method provides an active behavior of the bolts and gaskets undercritical conditions of internal pressures and a variety of operationaltemperatures. This active behavior may be considered as a new sealingphilosophy based on active intervention of the bolts and gaskets intocritical processes providing significant increase of the leakagetightness of the BFCs and maximizing the efficiency of pressure vesselsand piping systems used in critical engineering structures.

Changes may be made by those skilled in the art in matters of shape,size, and arrangement of the bolts, gaskets and flanges of the BFCswithout exceeding the scope of the invention described by appendedclaims.

1. A new sealing technology based on active intervention of bolts and gaskets of the Bolted Flanged Connections (BFCs) into technological processes of critical engineering structures by means of creep limitation or complete creep exclusion of bolts and gaskets.
 2. The new sealing technology according to claim 1 wherein said creep limitation or complete creep exclusion of said bolts and gaskets results from bolt contraction and gasket “swelling” at operational temperature of said BFCs.
 3. A method to limit or exclude a creep of bolts and gaskets of the BFCs wherein bolts and gaskets are manufactured from the same SMAs having temperature intervals of reverse martensitic phase transformation close to operational temperatures of the BFCs.
 4. The method according to claim 3 wherein said bolts and gaskets of said same SMAs are shape-memorized to the compression (bolts) and “swelling” (gaskets) during the bolt preload under temperature below than temperature of martensite state of the SMAs.
 5. The method according to claim 3 wherein said bolts and gaskets of said same SMAs are shape-memorized in advance to said compression (bolts) and “swelling” (gaskets) when the bolts and gaskets, being previously stretched (bolts) and compressed (gaskets) under temperature below than temperature of martensite state of the SMAs, obtain some quantity of residual elongations and contractions after unloading.
 6. The new sealing technology and method according to claims 1 and 3 wherein said bolts and gaskets manufactured from the same said SMAs and shape-memorized to said compression (bolts) and “swelling” (gaskets) display “negative creep” effect resulting from constrained recovery of initial length of said bolts and initial thickness of said gaskets followed by appearance of reactive shape-recovering stresses having direction inverse to the direction of operational creep of bolts and gaskets under said operational temperatures of the BFCs that are close to temperature intervals of reverse martensitic phase transformation of said SMAs from which said bolts and gaskets are manufactured. 